1. Field of the Invention
The present invention relates to surface acoustic wave elements for use in, for example, surface acoustic wave filters or the like, and more specifically, the present invention relates to a surface acoustic wave element that has a structure that prevents electrodes from being broken by the pyroelectric effect of the piezoelectric substrate.
2. Description of the Related Art
In a surface acoustic wave element, electrodes connected to different potentials of the interdigital transducer (IDT) may cause electric discharge due to: (a) electrical charges on the surface of the piezoelectric substrate resulting from the pyroelectric effect of the substrate produced by temperature changes; or (b) electrical charges induced by applying a surge voltage. If the amount of electric discharge is large, the electrodes are degraded or broken. Accordingly, the characteristics of the surface acoustic wave element are deteriorated.
In order to prevent the electric discharge from damaging the electrode, the specific resistance of the piezoelectric substrate can be reduced. A reduced specific resistance of the substrate allows electrical charges suddenly produced on the surface of the substrate to move on the surface of the substrate to reduce the potential difference rapidly. Thus, electric discharge resulting from local charge accumulation can be prevented. Consequently, the resistances to pyroelectric destruction and to electric power can be enhanced in a surface acoustic wave device using a piezoelectric substrate, such as a surface acoustic wave filter.
Some approaches for reducing the specific resistance of the substrate have been known in which the substrate may be doped with Fe or other carriers from the surface, or the substrate may be heated in a reducing atmosphere of a specific gas under reduced pressure.
For example, Japanese Unexamined Patent Application Publication No. 11-92147 has disclosed LiNbO3 and LiTaO3 crystals preconditioned so as to increase the ability to reduce surface charging and a method for preparing those crystals. More specifically, Japanese Unexamined Patent Application Publication No. 11-92147 has disclosed a method of heat-treating a LiNbO3 or LiTaO3 crystal in a reducing atmosphere at a temperature of 500° C. to 1,140° C. under reduced pressure. However, this publication has not described specific conditions of the reduced pressure in the reducing atmosphere. According to this method, a gas, such as argon, water, hydrogen, nitrogen, carbon dioxide, carbon monoxide, oxygen, or a mixture thereof is used for preparing the reducing atmosphere.
In general, in the manufacturing process of a surface acoustic wave device, some steps are performed in a high-temperature atmosphere containing oxygen. For example, in a manufacturing process of a chip-size-packaged surface acoustic device in which a surface acoustic wave element is mounted on a mount board by flip chip bonding and sealed with a resin, the step of the flip chip bonding and the step of thermally curing the resin are performed in high-temperature atmospheres containing oxygen. In a manufacturing process of a surface acoustic wave device in which a surface acoustic wave element is mounted on a ceramic package by flip chip bonding, the step of the flip chip bonding is performed in a high-temperature atmosphere containing oxygen. In a manufacturing process of a surface acoustic wave device including the step of mounting a surface acoustic wave element to a ceramic package by wire-bonding after die bonding, the step of thermally curing a die bonding agent is performed in a high-temperature atmosphere containing oxygen.
Although the specific resistance of the piezoelectric substrate used for the surface acoustic wave element is reduced by heat treatment under reduced pressure in a reducing atmosphere, as disclosed in the Japanese Unexamined Patent Application Publication No. 11-92147, the piezoelectric substrate is reoxidized if the surface acoustic wave element is exposed to a high-temperature atmosphere containing oxygen in the above-described manufacturing processes of the surface acoustic wave device. In the resulting surface acoustic wave device, therefore, the piezoelectric substrate is liable to experience the pyroelectric effect again, in spite of using the method of Japanese Unexamined Patent Application Publication No. 11-92147. Consequently, electric discharge occurs between electrodes connected to different potentials of the IDT, and degrades or damages the electrodes disadvantageously.